A crude fraction isolated from human serum has been demonstrated to inhibit significantly the ability of platelets to adhere to artificial surfaces such as those found in many artificial organs. Preliminary studies suggest that the inhibitory activity is associated with a low density lipoprotein fraction. We propose to isolate, purify and characterize this platelet adhesion inhibitor that has been found to be present in the plasma of all normal human subjects tested. This adhesion inhibiting activity is remarkably stable in serum or plasma but relatively unstable in various semipurified states. Attempts will be made to: (1) stabilize the activity, (2) purify and characterize the inhibitor, (3) determine the mechanisms whereby this component inhibits platelet adhesion, (4) develop a method of quantitation for this inhibitor in the absorbed state, and (5) at later stages of the proposed project investigate the level of this inhibitor in normal subjects as well as in persons who have suffered thrombotic complications coincident with the use of oxygenators, hemodialyzers, and artificial heart valves and vascular segments. Purification of this plasmatic factor may well have significant practical applications for use in artificial organs that interface with blood.